How to make a slipcover using but preserving the old one as a pattern

The biggest DIY project in my recent home office renovation was a sewing project! Back to my roots, I guess! I found a really nice slipcovered armchair on Facebook Marketplace for $40 and honestly, the slipcover was fine as-is, just neutral cotton canvas, but I wanted something definitely not neutral! 

I've made slipcovers before (like this one in 2014 and these in 2016) but I was definitely more confident doing it by cutting apart the previous slipcover and modifying it (or doing a simpler chair...